1. Field of the Invention
The present invention relates generally to automatic workpiece sensing, and relates specifically to apparatus and methods for sensing a workpiece with an electrical arc in order to adaptively position a tool in relation to the workpiece. The disclosed embodiment of the present invention is particularly adapted for use with tool manipulators such as multi-axis robots by obtaining information related to the topography of the workpiece so that the tool manipulator may adaptively position the tool as a function of the sensed topography of the workpiece.
2. Description of the Prior Art
Increased needs for automation in industry have resulted in the application of multi-axis manipulators, commonly called "robots", which move tools into proximity with a workpiece and perform a task. Tasks frequently performed include loading and unloading machinery, grinding, milling, spray painting, gluing, heat treating, assembling, welding, and the like.
The Robot Institute of America defines a "robot" as a "reprogrammable multifunctional manipulator designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks." Robots are most widely classified according to whether they are controlled by nonservomechanisms or servomechanism drives. In nonservomechanism drives, the robot's tool center stops only at fixed-end points of motion. Although many sequential motions are possible along each axis, such motions can be accomplished only between two specific end points; there is therefore no motion control between points, and acceleration and deceleration of the arm is not possible. Simple pick-up-and-place robots fall within this class.
On the other hand, servomechanism-controlled robots have continuous-path arm motions and can be programmed to stop at any point within the arm's range of motion. Acceleration and deceleration of the arms is also possible. These robots, however, require sensors to gather feedback information concerning the velocity, position, acceleration, force and torque status of the manipulator. The servomechanism system compares the information obtained from the sensor with predetermined operational parameters in the control program. When deviations from the predetermined parameters are detected, the robot's servomechanism initiates corrective action.
Present research in sensor technology is directed in such diverse areas as vision-sensing systems (which includes such techniques as television-like array cameras as well as scanning fiber-optic vision systems and laser reflecting systems), tactile sensors, and sound-reflecting or sonar sensing systems. Sensing systems may be broadly classified in two categories: contacting sensors and noncontacting sensors. Contacting sensors include tactile, force array, and other devices which actually touch the workpiece being sensed. Noncontacting sensors include vision, eddy current, laser, acoustic and welding arc sensors.
A principal application of sensors to the movement of tool manipulators is to allow the manipulator, during operation, to adapt to prevailing and detected conditions rather than the ideal conditions originally programmed. Paths of travel, position of programmed points of movement, speed control, and timing are variables which appropriate sensors will allow to be adaptively controlled. For example, a scanning function would allow the topography of the workpiece to be mapped in three dimensions so that the features of the workpiece such as corners, holes, curvature, and the like can be located. Adaptive speed control enables the robot to select feed rates for hard surfacing, deburring, grinding, and similar operations. Contour tracking features enable the robot to follow a path which is not clearly defined and which deviates from the programmed path, as when the workpiece is defective or out of tolerance. Adaptive time and speed control furthermore assist the optimization of cycle times. Appropriate sensors coupled with multi-axis manipulators thus allow the creation of a computer map of the workpiece, recognition of the workpiece, planning or programming of actions to carry out an assigned task, recover from mistakes or anomalies encountered in the workpiece, and recognize when the assigned task is completed.
Although much emphasis is presently being given to vision sensing sensors, techniques for implementing vision sensing depend havily upon research in disciplines such as computer science, artificial intelligence, pattern recognition, and the like. Present vision oriented or electro-optical systems require extensive computing power in order to effectively and efficiently function. As a consequence, most present vision oriented systems, which frequently involve the use of complex signal processing algorithms, are either extremely simplistic or slow, or are very sophisticated and costly, as well as difficult to maintain reliably.
Prior to the present invention, it had been thought that techniques for magnetic and mechanical arc deflection for use in connection with arc sensors in nonwelding applications suffered from similar disadvantages. For example, it was once thought that arc sensors required distinct features of the workpiece such as sidewalls to be present for the sensor to differentiate between areas of the workpiece. It has now been discovered that an electrical arc may be successfully employed to sense the topography of a workpiece without the requirement that predefined sharp features such as a sidewall be present.